Calystegia sepium – an expansive weed of maize fields near Krakow

The aim of the present study was to assess the degree of colonization of maize fields by the greater or hedge bindweed, Calystegia sepium L. in the vicinity of Krakow, and to investigate the habitat preferences of this weed. On the basis of field trips, carried out in 88 fields of maize, we found that C. sepium had colonized approximately 30% of the fields investigated. On average, it covered 17.5% of the field area. Multivariate analyses (PCA and RDA) showed that the presence of this species was correlated with that of sandy soils as well as the proximity of streams. Analysis of the morphological features revealed that C. sepium specimens growing both in fields and in natural stands share similar morphological characteristics.


Introduction
Greater or hedge bindweed (Calystegia sepium L.) is a twining perennial belonging to the family Convolvulaceae.In Poland, the typical habitat of this species, which prefers growing in riverine shrubs [1], is called a "veil community".Calystegia sepium is closely related to field bindweed (Convolvulus arvensis L.), which is considered to be one of the world's most aggressive weeds [2].In some European countries, C. sepium is also considered a troublesome weed [3,4], especially as it affects perennial crops and orchards [3].European farmers have recently reported that C. sepium has become an increasingly important weed of maize fields [5].Calystegia sepium poses a significant threat to maize crops throughout France and the Netherlands (widespread and regular populations of C. sepium), though less widespread and more sporadic populations of this species occur in southwestern Germany, and regional or rare populations in southern Poland [6,7].Interestingly, C. sepium affects maize irrespective of the prevailing soil tillage system, whether it is chisel, disk harrowing, or moldboard plough [8].
Calystegia sepium significantly impedes mechanical harvesting [3].Furthermore, it is a source of biological threat to crops, being a host for herbivorous insect pests and viruses that cause plant diseases [9].
Mechanical and chemical management of C. sepium is difficult, due to its massive regenerative potential [5].Moreover, efforts to destroy this weed by tilling may result in the spreading of its rhizomes throughout the field, resulting in both vegetative reproduction and dispersal [4].For chemical control of C. sepium, both selective and non-selective herbicides are applied, e.g., 2,4-D, dicamba, glyphosate, imazapyr, but these do not eradicate the plants.Instead, they merely reduce the growth and number of shoots and roots produced [9].Recently, several pieces of research have been carried out on the biological control of C. sepium using pathogenic fungi [3,4,9,10].
An increase in the area used for growing maize in Poland [11] has necessitated an evaluation of the problems caused by C. sepium [7].Polish scientific literature lacks information about the frequency of C. sepium in maize fields, even on a regional scale.Having considered the previously mentioned threats posed by C. sepium, this work focuses on: (i) the assessment of C. sepium colonization of maize fields near Krakow, (ii) the recognition of the habitat preferences of this weed, and (iii) comparison of selected morphological traits of C. sepium collected from maize fields and from adjacent natural stands.

Material and methods
Twelve field trips were performed in the fall of 2010-2013 in five physico-geographical mesoregions in the vicinity of Krakow City, namely: Tenczynek Prominence, Skawiński Trench, Olkuska Bernese, Wieliczka Foothills, and Proszowicki Plateau (Fig. 1).In total, 88 maize fields were surveyed, and in each case the following characteristics were determined: (i) field area (ha), (ii) field slope (degrees), (iii) presence of streams, and (iv) the use of herbicides.Streams were considered to be present in the vicinity of a field if they were located no further than 50 m from the field margin.The effect of herbicide was recorded based on its effect on the degree of colonization by C. sepium and the symptoms caused by applying it to this weed.
Where present in a field, the quantity of C. sepium was determined using the Braun-Blanquet scale [12], and the results were later expressed as the average percentage area cover.At each location for C. sepium, soil samples were collected from five different sites per field at a depth of 30 cm using a soil auger.Soil samples were also collected from selected fields where no C. sepium was observed for comparison of soil conditions between stands.The soil samples were dried at room temperature and then sieved using a 2 mm mesh sieve.The prepared soil material was further used to determine soil texture by the aerometric method (according to Polish standard PN-R-04032 [13]) and soil pH in 1 M solution of KCl [14].
The above data were subjected to multivariate analysis in order to correlate the presence of C. sepium with selected habitat characteristics.The principal component analysis (PCA) was performed for all 27 stands where C. sepium was noted using PQ Stat ver.1.6 (PQStat Software, Poznań, Poland).A Kaiser-Meyer-Olkin (KMO) coefficient was calculated using Canoco for Windows ver.4.51 (Biometrcis Plant Research International, Wageningen University and Research, the Netherlands), indicating the correlation of the primary variables.A redundancy analysis (RDA) was conducted for both stands with and without C. sepium.
For sampling from each of the four locations, two specimens of C. sepium were collected from a maize field and an adjacent natural stand.Plants were collected with rhizomes.For each specimen, the following morphological features were measured: number and length of shoots (cm), number of seeds per capsule, length (cm), and diameter of rhizomes (mm).The results of morphological measurements were analyzed statistically using one-way ANOVA with STATISTICA PL ver.10.0.

Results
In the studied area, Calystegia sepium was present in 27 of the 88 fields containing maize, which represented less than 30% (Tab.1).The most frequent occurrence was recorded for the Skawiński Trench (nine locations), especially where maize fields were located close to the Vistula River, ditches, or local streams.Calystegia sepium appeared least frequently in maize fields of the Proszowice Plateau (two locations).Most probably, the difference in C. sepium frequency between these two regions is due to differences in the management regimes of agricultural land.In the Skawiński Trench, fields are rather small and herbicides are used less frequently compared with the Proszowice Plateau.Furthermore, the soil here is good and the crops grown are mostly vegetables.
The average percentage area occupied by C. sepium ranged from 0.5% to 87.5%, but mainly this was approx.17.5%, which corresponds to a value of "2" on the Braun-Blanquet scale (Tab.1).Frequently, C. sepium was found on clayey silt and silt loam having a wide range of pH values ranging from 4.2 to 7.3.The lowest pH values were observed in the Skawiński Trench soils (Tab.1).
PCA analysis of the basic features of habitats inhabited by C. sepium explained 50.54% of the total variance, the KMO coefficient value being 0.39.Fig. 2 shows the variables responsible for the presence of C. sepium in maize fields.According to PCA analysis, the presence of sand in the soil of maize fields was particularly important for colonization by C. sepium.Conversely, RDA analysis explained 80% of the total variance and also confirmed that the sand fraction played a significant role relative to the presence of C. sepium.Furthermore, it revealed a positive correlation between the presence of the weed and the proximity of a stream as well as herbicide application (Fig. 3).Interestingly, RDA analysis showed that the presence of C. sepium was positively correlated with a particular soil texture, namely, clayey silt, even though each soil fraction (silt and clay) analyzed separately was not correlated with the presence of C. sepium in the field (Fig. 3).
RDA analysis revealed that the application of herbicide increased the frequency of C. sepium in maize fields (Fig. 3).It was also observed that regenerated C. sepium plants had a broader possibility to colonize maize if other weed species were destroyed by herbicide treatment, but this requires further, more detailed investigation.
Analysis of variance carried out for the selected morphological traits of C. sepium plants collected from maize fields and from adjacent natural stands showed that the plants do not differ statistically in terms of morphology (p = 0.3-0.7;Tab. 2).The analyzed plants had, on average, more than four shoots, each approx.200 cm in length.The average length of rhizomes was 40 cm, and their diameter ca.6 mm.The number of capsules per shoot was 0-5, each containing ca. 3 seeds (Tab.2).
Tab. 1 Characteristics of maize fields where observations were made, and the frequencies of C. sepium based on the Braun-Blanquet (B-B) [12] scale for each field.The same analysis also revealed that the application of herbicide increased the frequency of C. sepium in maize fields.Many authors [3,4,8,10] have reported that C. sepium is difficult to manage using herbicides.This is due to its deep-growing rhizomes which are difficult to destroy with chemicals and which are a source of new regenerating plants.

Field
Analysis of variance carried out for the selected morphological traits of C. sepium plants collected from maize fields and from adjacent natural stands showed that the plants do not differ statistically in terms of morphology.This lack of morphological differences between plants obtained both from their natural habitat and the maize field habitat indicates that this species is highly adaptable to new conditions.Currently, C. sepium poses a local threat to maize fields [6,7,16], but owing to the increasing popularity of maize growing in Poland, this species may soon become a pernicious weed.Rask and Andreasen [3] listed C. sepium as a troublesome weed   that is as expansive as C. arvensis.A systematic survey of maize fields in order to assess the capacity of C. sepium as a weed in other regions of Poland is certainly recommended.
Summing up, we found that Calystegia sepium had colonized approximately 30% of the fields investigated.Mostly, the weed was found on rather small fields, on clayey silt and silt loam, regardless of soil pH, where herbicide control was less frequent.On average, C. sepium covered 17.5% of the field area.Multivariate analyses (PCA and RDA) showed that the presence of this species is correlated with that of sandy soils as well as the proximity of streams.Analysis of the morphological features revealed that C. sepium specimens growing both in fields and natural stands share similar morphological characteristics, which might suggest a good adaptability of C. sepium to the conditions of maize crops.

Fig. 1
Fig. 1 Location of investigated maize fields in the vicinity of Krakow.

Fig. 2
Fig. 2 PCA diagram displaying the main variables that determine the presence of Calystegia sepium amongst maize crops near Krakow.The upper right quadrant shows the most important variables.The importance of variables diminishes clockwise.Variance explained by Factors 1 and 2 is presented on the graph.

Fig. 3
Fig. 3 RDA diagram presenting the main variables that determine the presence of Calystegia sepium in maize fields.The upper quadrants represent the variables positively correlated with the presence of C. sepium, whereas the lower quadrants show the important variables associated with the absence of C. sepium in a field.Variance explained = 80%; F-ratio = 101.1;p = 0.002.
[9]the field.Ivanova et al.[17]noted the presence of C. sepium in the sandy and saline soils of Pomorie in Bulgaria.Pfirter[9]also confirmed that C. sepium prefers moist soils.Contrary, farming magazines state that moist habitats rich in nutrients and clay are typical habitats for C. sepium[16].All these data may indicate quite broad soil preferences of C. sepium, under one condition -high moisture content. sepium 2ab.2Selected morphological features of Calystegia sepium plants collected from four natural stands and four adjacent maize fields (mean value ±SE)